Apparatus for fabricating a glass rod and method of same

ABSTRACT

The present invention provides a apparatus and a method for fabricating a glass rod capable of suppressing a diameter fluctuation of a drawn glass rod even in case of a relatively large diameter reduction ratio between a glass preform and a glass rod, such as 60 to 95%. A feed speed V 1  of the glass preform is set to a constant value, a diameter D of the glass preform is acquired for determining the drawing speed V 2  from a measured diameter data of the glass preform before being drawn at a diameter acquisition position defined with respect to a reference position of the furnace, and a distance from the reference position to the diameter acquisition position is defined so as to vary depending on a diameter fluctuation of the glass preform before being drawn in a longitudinal direction thereof.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of Japanese Patent Application Nos.2010-200504, filed Sep. 8, 2010 and 2011-194105, filed Sep. 6, 2011which are hereby incorporated by reference herein in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an apparatus for fabricating a glassrod and a method of the same, in particular, to an apparatus and methodfor fabricating a glass rod having a desirable diameter by feeding arelatively large diameter glass preform such as an optical fiber glassingot in a furnace, heating the preform in the furnace and drawing theheated preform from the furnace.

2. Description of the Related Art

Japanese Patent Laid-Open No. 2006-193397 discloses a method forfabricating a glass rod having a desirable diameter by measuring adiameter of a preform during drawing at a region where a deformation(diameter reduction) is progressing and a diameter at a region where thediameter reduction is almost completed, and adjusting a feed speed and adrawing speed of the glass preform with respect to a furnace based onthese measured diameters.

The description of conventional art in Japanese Patent Laid-Open No.H11-011970 (1999) a method of pre-measuring a diameter of a glasspreform along a longitudinal direction thereof, determining a ratiobetween a feed speed and a drawing speed of the preform, and fabricatinga constant diameter glass rod based on the based on the ratio.

Japanese Patent Laid-Open No. 2006-219331 discloses suppressing adiameter fluctuation of a drawn glass rod caused by a shifting of areference position defining a feed speed and a drawing speed of a glasspreform as a drawing process progress, by changing the amount of a feedand the reference diameter distance of the glass preform.

Conventionally, a ratio of a target diameter of a glass rod with respectto a diameter of a glass preform (referred to as a diameter reductionratio below) has been about 20% to 50% and relatively small.Accordingly, the control method of Japanese Patent Laid-Open No.2006-193397 could suppress the fluctuation of diameter to a requiredlevel. Recently, however, a larger size optical fiber preform isrequired, and a glass rod having a relatively small diameterdeformation, in which a diameter reduction ratio is about 60% to 95%, isrequired. For example, when a 160 mm to 170 mm diameter glass preform isdrawn into a 150 mm diameter glass rod, the diameter reduction ratio is88% to 94%.

To implement the feed back control disclosed in Japanese PatentLaid-Open No. 2006-193397, it is necessary to measure a diameter ataround a position where a diameter is substantially reduced in adiameter decreasing region. In case of a relatively large diameterreduction ratio, however, a position where a diameter is substantiallyreduced is adjacent the heater in a furnace. Accordingly, it isdifficult to directly measure a diameter at this position. If thediameter used for the feedback control is measured at a location spacedfrom the heater to some extent so as to prevent an affection of theheater, the response of the feedback control could be lagged. Thus thefeed back control may not be appropriately implemented. As a result, alarge fluctuation can be generated in a drawn glass rod.

According to the method disclosed in Japanese Patent Laid-Open No.H11-011970 (1999), a relatively desirable diameter fluctuation value canbe obtained even at a diameter reduction ratio of 60% to 95% in the caseof a stable constant diameter glass preform. The method, however, maycause an unacceptable diameter fluctuation (specifically, more than ±1%)at an end portion of a usable region in a drawn glass rod at the end ofthe drawing process, when the glass preform has a relatively largediameter fluctuation in a longitudinal direction thereof.

The method disclosed in Japanese Patent Laid-Open No. 2006-219331 cansuppress a diameter fluctuation of a glass rod. In the method, however,a criterion for changing the reference diameter position is indefinite,and an unacceptable diameter fluctuation may be generated depending on acondition of a diameter fluctuation of a glass preform. In addition, inthe embodiment of the publication, a 130 mm diameter glass preform isdrawn into a 30 mm diameter glass rod, that is, a diameter reductionratio is considerably small such as 23%. The publication fails todisclose a method for suppressing a diameter fluctuation of a drawnglass rod in case of a relatively large diameter ratio such as 60 to95%.

SUMMARY OF THE INVENTION

The present invention provides an apparatus and a method for fabricatinga glass rod capable of suppressing a diameter fluctuation of a drawnglass rod even in case of a relative large diameter reduction ratiobetween a glass preform and a glass rod, such as 60 to 95%.

A first aspect of the present invention provides a method of fabricatinga glass rod, the method feeding a relatively large diameter glasspreform into a furnace through a top portion thereof and drawing theglass preform from the furnace through a bottom portion thereof so thatthe relatively large diameter glass preform is drawn into a relativelysmall diameter glass rod, including the steps of:

controlling a feed speed (V1) and a drawing speed (V2) of the glasspreform so that a ratio (V2/V1) between the feed speed (V1) and thedrawing speed (V2) becomes a value ((D/d)²) determined based on adiameter (D) of the glass preform and a target diameter (d) of the glassrod;

setting the feed speed (V1) of the glass preform to a constant value;and

acquiring the diameter (D) of the glass preform for determining thedrawing speed (V2) from a measured diameter data of the glass preformbefore being drawn at a diameter acquisition position defined withrespect to a reference position of the furnace, wherein

a distance from the reference position to the diameter acquisitionposition is defined so as to vary depending on a diameter fluctuation ofthe glass preform before being drawn in a longitudinal directionthereof.

A second aspect of the present invention provides an apparatus forfabricating a glass rod, including:

a furnace;

a feeding mechanism configured to feed a relatively large diameter glasspreform into a furnace through a top portion thereof;

a drawing mechanism configured to draw the glass preform from thefurnace through a bottom portion thereof so that the relatively largediameter glass preform is drawn into a relatively small diameter glassrod;

a controller configured to control a feed speed (V1) of the glasspreform by the feeding mechanism and a drawing speed (V2) of the preformby the drawing mechanism so that a ratio (V2/V1) between the feed speed(V1) and the drawing speed (V2) becomes a value ((D/d)²) determinedbased on a diameter (D) of the glass preform and a target diameter (d)of the glass rod, wherein

the controller comprises:

a setting unit configured to set the feed speed (V1) of the glasspreform to a constant value;

an acquisition unit configured to acquire the diameter (D) of the glasspreform for determining the drawing speed (V2) from a measured diameterdata of the glass preform before being drawn at a diameter acquisitionposition defined with respect to a reference position of the furnace,wherein

a distance from the reference position to the diameter acquisitionposition is defined so as to vary depending on a diameter fluctuation ofthe glass preform before being drawn in a longitudinal directionthereof.

According to the present invention, a diameter for determining a drawingspeed is acquired from a measured diameter data of a glass preformbefore being drawn, and a distance from a reference position of a heaterwith respect to a diameter acquisition position for acquiring thediameter is adjusted depending on a diameter fluctuation of the glasspreform. Thereby, the diameter acquisition position can be modifieddepending on a diameter reference distance, which changes in accordancewith the diameter fluctuation of the glass preform. As a result, adrawing speed can be determined based on a diameter responding to thedrawing reference distance, so that a diameter fluctuation of a drawnglass rod in a longitudinal direction of the glass rod can besuppressed.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments (with reference to theattached drawings).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view schematically showing an apparatus for fabricating aglass rod according to an embodiment of the present invention;

FIG. 2 is a schematic view for explaining a drawing reference distancein a diameter decreasing region;

FIG. 3A is a graph showing a relation between a drawing referencedistance and a feed speed in case where 160, 170 and 180 mm diameterglass preforms are drawn into 150 mm target diameter glass rods;

FIG. 3B is a graph showing a relation between a drawing referencedistance and a feed speed in case where 160, 170 and 180 mm diameterglass preforms are drawn into 110 mm target diameter glass rods;

FIG. 4 is a flow chart showing an example of a drawing processing by thecontroller of FIG. 1;

FIG. 5 is a graph showing a diameter fluctuation of a glass preformbefore and after drawing in Example 1;

FIG. 6 is a graph showing a diameter fluctuation of a glass preformbefore and after drawing in Comparative example 1;

FIG. 7 is a graph showing a diameter fluctuation of a glass preformbefore and after drawing in Comparative example 2; and

FIG. 8 is a graph illustrating a diameter fluctuation of a glass preformbefore and after drawing.

DESCRIPTION OF THE EMBODIMENTS

An embodiment of the present invention will be described below withreference to the attached drawings. FIG. 1 schematically shows anapparatus for fabricating a glass rod according to an embodiment of thepresent invention. The fabricating apparatus has a feeding mechanism 101and a drawing mechanism 110, which are disposed on a frame FR verticallyextending, respectively, a furnace 120 disposed on the frame between thefeeding mechanism 101 and the drawing mechanism 110, and a controller130.

The feeding mechanism 101 has a screw shaft 102 vertically extending androtatably supported, a motor 103 for driving the screw shaft 102, amovable member 104 into which the screw shaft 102 is screwed, and achucking mechanism 105 which is disposed on the movable member 104 andholds an upper end portion of an optical fiber glass preform 201.

The drawing mechanism 110 has a screw shaft 112 vertically extending androtatably supported, a motor 113 for driving the screw shaft 112, amovable member 114 into which the screw shaft 112 is screwed, and achucking mechanism 115 which is disposed on the movable member 114 andholds a lower end portion of the optical fiber glass preform 201.

The furnace 120 has an annular shaped of heater 121 therein, and heatsan optical fiberglass preform 201 passing through a central portion ofthe heater 121.

The controller 130 is constituted of hardware such as a processor and amemory, and required software, and is electrically connected to themotors 103 and 113, and the furnace 120. Specifically, the controller130 controls rotational velocities of the motors 103 and 113, and atemperature in the furnace 120.

Drawing of a glass preform by the apparatus in FIG. 1 will be described.First, a glass preform 201 having a relatively large diameter is fedinto the furnace 120 through the top portion thereof by the feedingmechanism 101. The glass preform 201 fed into the furnace 120 is drawnfrom the furnace 120 through a bottom portion thereof by the drawingmechanism 110 so that the glass preform 201 is drawn so as to be arelatively small diameter glass rod. At that time, the feeding mechanism101 and the drawing mechanism 110 are controlled so that a ratio V2/V1between a feed speed V1 and a drawing speed V2 of the glass preform 201becomes a value (D/d)² which is defined from the glass preform diameterD and the glass rod target diameter d. That is, a feed speed V1 and adrawing speed V2 are controlled so that a relation defined by thefollowing formula (1) is satisfied.

V2/V1=(D/d)²  (1)

In the present embodiment, a feed speed V1 of the glass preform is setto a constant value for a drawing control. As the glass preform diameterD for determining the drawing speed V2, the preform diameter before theglass preform is drawn at a diameter acquisition position which isdefined with respect to a reference position of the furnace from ameasured diameter data which is obtained to measure the glass preformalong a longitudinal direction of the glass preform. A distance from areference position in the furnace to the diameter acquisition positionis determined so as to vary depending on the glass preform diameterfluctuation before being drawn in a longitudinal direction thereof. Inparticular, the distance from a reference position in the furnace to thediameter acquisition position is determined so as to be equal to adrawing reference distance, which will be described later.

Next, a drawing reference distance will be discussed with reference toFIG. 2. The glass preform 201 is fed into the furnace 120 through thetop portion of the furnace 120 at a feed speed V1, heated by the heater121, and drawn from the furnace 120 through the bottom portion of thefurnace 120. Here, a tension is induced on the glass preform 201 bysetting V2>V1. A heated and softened region of the glass preform 201 isstretched due to the tension so that a diameter decreasing region 202where a diameter is gradually reduced in a longitudinal direction isformed. A diameter of the glass preform 201 decreases in the diameterdecreasing region 202 thereby a relatively small diameter glass rod 203is formed.

The glass preform 201 fed into the furnace 120 is heated by the heater202 so that a temperature of the preform 201 in the longitudinaldirection reaches a maximum temperature at a position below a middleposition C of the heater 121 and gradually decreases from the maximumtemperature position downward. Accordingly, a position PM where adeformation rate (an amount of diameter reduction per unit length in thelongitudinal direction) is largest is located at below the heater middleposition C at any time.

Here, VM indicates a volume of a glass preform 201 from the heatermiddle position C to the position PM, ND indicates a diameter of theglass preform 201 before being drawn, L indicates a drawing referencedistance. In the present embodiment, a drawing reference distance L isdefined by the following formula (2).

L=VM/(n×(π/2)²)  (2)

A drawing reference distance L is defined based on a distance from theheater middle position C to the position on the glass preform beingdrawn, and varies depending on a feed speed and a diameter of a glasspreform before being drawn, as described later. That is, a drawingreference distance L is a distance between the heater middle position Cand a specific position which is defined depending on a deformationcondition of the diameter decreasing region on the glass preform.Accordingly, a drawing reference distance L is depending on the position1M in the diameter decreasing region 202, where the deformation rate islargest.

In the present embodiment, a diameter of the glass preform correspondingto a drawing reference distance is used for the drawing control. Bycontrolling the diameter of the glass preform corresponding to thedrawing reference distance, a diameter of a drawn glass preform can becontrolled with higher precision. The diameter fluctuation region,however, is located close to the heater 121 so that it is difficult todirectly measure its shape during drawing. Accordingly, in the presentembodiment, the drawing reference distance can be calculated orestimated from a preliminary experiment or data when being drawn in asteady state.

The present inventor investigated a relation between a drawing referencedistance L, a feed speed V1 and a diameter D of a glass preform beforebeing drawn. In particular, a glass preform diameter D, a glass rodtarget diameter d and a feed speed V1 were set to a variety of values,and a drawing reference distance L was measured from an actual shape ofa diameter decreasing region 202. The results are shown in FIGS. 3A and3B. FIG. 3A shows a relation between a drawing reference distance and afeed speed in a case where 160, 170 and 180 mm diameter glass preformswere drawn into 150 mm target diameter glass rods, respectively. FIG. 3Bshows a relation between a drawing reference distance and a feed speedin a case where 160, 170 and 180 mm diameter glass preforms were drawninto 110 mm target diameter glass rods. As can be seen from the relationbetween a feed speed and a glass preform diameter with respect to aplurality of drawing reference distances, a drawing reference distance Lvaries depending on a glass preform diameter and feed speed.

It can be seen from the relation between a feed speed and a glasspreform diameter with respect to a plurality of drawing referencedistances shown in FIGS. 3A and 3B, a drawing reference distance variesdepend on a glass preform diameter and a feed speed in various way. Thebigger the glass preform diameter before being drawn, the longer thedrawing reference distance. And, the greater the feed speed, the longerthe drawing reference distance.

As can be seen from FIG. 3A and FIG. 3B, keeping a feed speed and aglass preform diameter during drawing at constant values can provides aconstant length of a drawing reference distance. However, if a diameterfluctuation of the glass preform in a longitudinal direction thereofexists, a drawing reference distance may be fluctuated even thoughkeeping the feed speed of the glass preform constant. For this reason,in the present embodiment, a feed speed is kept constant during drawingof a glass preform and a diameter acquisition position on the glasspreform is changed depending on the diameter fluctuation of the glasspreform. That is, in the present embodiment, a distance from a heaterreference position to a diameter acquisition position of the glasspreform is set so as to be equal to a fluctuating drawing referencedistance.

To determine a drawing reference distance, data defining a relationshipbetween a diameter of a glass preform before being drawn and a drawingreference distance, as shown in FIGS. 3A and 3B can be used.

It is difficult to prepare drawing reference distance data with respectto all of the fluctuated diameters of the glass preform. For thisreason, in the present embodiment, by interpolating between data in thediameter and drawing reference distance data as shown in FIGS. 3A and3B, a drawing reference distance is determined at a given position onthe glass preform in a longitudinal direction of the glass preformthereby determining a diameter acquisition position when the givenposition reaches the heater middle position C.

In a case where a diameter of the usable region in a glass preformsubstantially linearly varies, drawing reference distances at two pointsof a start position and a drawing ending position of the usable regionis determined using diameter drawing reference distance data. And, bylinearly interpolating the drawing reference distance at these twopoints, a drawing reference distance at a given position between the twopoints can be calculated. That is, in a case where a glass preformdiameter before being drawn is linearly varied, a drawing referencedistance at a given position between the two points is also linearlychanged. In addition, in a case where a glass preform diameter does notvary in the longitudinal direction, a drawing reference distance at agiven position can be obtained by dividing a usable region into aplurality of interpolation regions, determining drawing referencedistances at both ends points of each of the interpolation regions fromthe diameter drawing reference distance data, and interpolating thedrawing reference distance at both ends points of each of theinterpolation regions. Note that an interpolating manner is not limitedto this manner, and another interpolating manner can be employed.

Next, an example of a drawing process by the above controller will bedescribed with reference to FIG. 4. First, a glass preform 201 of whichdiameter having pre-measured is set to the apparatus of FIG. 1, and theheater 121 is switched on and the motors 103 and 113 are driven to feedthe glass preform 201 into the furnace 120 (S1).

If starting to heat the glass preform from a drawing start position of ausable region of a glass preform which can be used for forming the glassrod, a diameter fluctuation may be generated on the drawing start sideof the preform because a temperature distribution of the preform is notyet in a steady state. To prevent this, it is necessary to start to heatthe glass preform so as to make a temperature distribution at thedrawing start position steady before a drawing start position of theusable region of the glass preform reaches the heater middle position Cin furnace 120. A length of a region between a position to start heatingthe glass preform and the drawing start position of the usable region ofthe glass preform (which is referred to as a preliminary heating regionbelow) is preferably set to be equal to or more than a length of theheater from a point of view to stabilize a temperature distribution.Extending it more than necessary is disadvantageous in productionefficiency because of a large loss of the glass preform, so it ispreferably set to less than three times of the length of the heater.

Next, acquisition of a current position of the glass preform 201 in thelongitudinal direction relative to the furnace 120 is started (S2). Thecurrent position of the glass preform 201 relative to the heater 121 canbe acquired using, for example, a rotational position detector (notshown in the figures) incorporated in the motor 103. Next, whether thedrawing start position of the glass preform 201 reaches the positionwhere is spaced from the heater middle position C by a predetermineddistance which is set to a specific drawing reference distance isjudged. If it reaches there, a drawing processing is started (S4).

With a drawing process being started, a feed speed V1 is set to apredetermined constant value (S5). Next, a diameter acquisition positionof the glass preform before being drawn is determined (S6). Here, amethod of determining a diameter acquisition position will be described,for example, in a case where a length of a usable portion in a glasspreform is 1000 mm, a drawing reference distance at a drawing startposition is 40 mm, and a drawing reference distance at a drawing endingposition is 60 mm. The drawing start position of the usable region inthe glass preform is set to a position below the heater middle positionC by 40 mm. A location of the drawing ending position with respect tothe heater middle position C when a diameter at the drawing endingposition is acquired is positioned below a location of the drawing startposition with respect to the heater middle position C by 20 mm (=60−40).Accordingly, a total amount of feed of the glass preform is set to 1020mm. And, multiplying an actual amount of feed of the glass preform by1000/1020 gives an interpolated diameter acquisition position.

With the drawing processing being started, a diameter at a calculateddiameter acquisition position is acquired from the measured diameterdata (S7). A drawing speed V2 is determined using the acquired diameterby the formula (1) (S8). And, a control command in accordance with thedetermined feed speed V1 and drawing speed V2 is sent to the motors 103and 113 (S9).

Next, whether the glass preform reaches an ending position is judged(S10). If it does not reach there, the steps S5 to S8 are repeated. Ifit reaches there, the drawing processing is terminated (S11).

The method of the present embodiment provides a great technical effectespecially in a case where the glass rod target diameter d is from 60 to95% of the glass preform diameter D. As can be seen from a comparisonbetween FIG. 3A and FIG. 3B, an amount of change of drawing referencedistance associated with variation of a feed speed and a diameterbecomes relatively larger in a drawing process with a relative largediameter reduction ratio in which a 160 mm diameter glass preform isdrawn into a 150 mm diameter glass rod and the ratio is 94%.Specifically, in the case of such a relatively large diameter reductionratio, the present invention is so useful in drawing. Also, a change ofdrawing reference distance associated with a variation in a glasspreform diameter can be seen even in the drawing of a relative smalldiameter reduction ratio such as from a 180 mm diameter to a 110 mmdiameter as shown in FIG. 3B, however, the amount of the change isrelatively small. In a case of less than 60% of a diameter reductionrate, a conventional method can be used, and the method of the presentinvention also can provide a desirable result. On the other hand, incase of over 95% of a diameter reduction rate, it is difficult tomaintain an appropriate drawing load during drawing so that a drawnglass rod may have some deflection.

Conventionally, if a diameter fluctuation of a glass rod when thediameter is reduced in a drawing furnace becomes large, the diameterfluctuation can be modified by re-drawing using an existing glass lathe.However, in a case of where a glass rod diameter is more than 110 mm, itbecomes difficult or impossible to re-drawing because heat efficiencyfalls in re-drawing using the existing glass lathe. Accordingly, in acase where a glass rod target diameter is over 110 mm, a glass rodhaving a decreased diameter fluctuation can not be fabricated without adrawing process of the present invention. In such case, the presentinvention provides effectiveness.

Example 1

A glass preform having a tapered portions at both ends thereof, a 1000mm length of usable region thereof, a diameter of 160 mm at a drawingstart position of the usable region, a diameter of 172 mm at a drawingending position of the usable region, and a diameter in the usableregion linearly varying in the longitudinal direction was drawn underconditions where a heater length is 130 mm, a glass rod target diameteris a 150 mm, and a feed speed is 10 mm/min. A drawing reference distancewas determined from the data of FIG. 3A, thereby drawing referencedistances were set to 33 mm and 52 mm on the drawing start side and onthe drawing ending side, respectively. A 200 mm length of thepreliminarily heated region was set on the drawing start side. The glasspreform was set so that a drawing start position of the usable region onthe drawing start side was located above the heater middle position C by167 mm and drawing was started at a heater temperature of 2050 degreesCelsius. A diameter portion larger than 150 mm of the tapered portion onthe drawing start side was drawn at a drawing speed calculated using theformula (1) while a target diameter of the portion was set to the 150 mmglass rod target diameter.

A feed amount of the glass preform was set to 0 mm at the time pointthat the 200 mm length of preliminarily heated region passed through theheater middle position C. From here, the glass preform was moved 1019mm. The feed amount of 1019 mm was determined from the usable regionlength of 1000 mm+(the ending position drawing reference distance of 52mm−the start position drawing reference distance of 33 mm). During thistime, a diameter acquisition distance with respect to a given positionon the preform was interpolated by multiplying an actual feed amount ofthe glass preform by 1000/1019. And, a drawing speed was calculated bythe formula (1) using a diameter acquired at the interpolated diameteracquisition distance from the measured diameter data of the glasspreform. In addition, another 200 mm length region was further drawnafter a drawing of the usable region. During this time, a feed speed wasmaintained at 9.0 mm/min, which is a final feed speed of the usableregion. The larger diameter portion than 150 mm of the tapered portionon the drawing ending side was drawn at a drawing speed calculated usingthe formula (1) while a target diameter of the portion was set to the150 mm. As a result, as shown in FIG. 5, a glass rod having asignificantly reduced diameter fluctuation, in which a differencebetween a maximum diameter and a minimum diameter is approximately 0.9mm, could be obtained.

Comparative Example 1

A glass preform having tapered portions at both ends thereof, a 1000 mmlength usable region of the glass preform, a diameter of 160 mm at adrawing start position of the usable region, a diameter of 170.5 mm at adrawing ending position of the usable region, and a diameter in theusable region linearly varying in the longitudinal direction was drawnunder conditions where a heater length is 130 mm, a glass rod targetdiameter is a 150 mm, and a feed speed is 10 mm/min. A preliminarilyheated region on the drawing start side was set to 200 mm in length. Adrawing reference distance was set to 41 mm determined from the data ofFIG. 3A by using a diameter of 165 mm at a middle position of thepreform as a representative diameter and was constant during thedrawing. The glass preform was set so that a drawing start position ofthe usable region on the drawing start side was located above the heatermiddle position C by 159 mm and a drawing was started at a heatertemperature of 2050 degrees Celsius. A diameter portion larger than 150mm of the tapered portion on the drawing start side was drawn at adrawing speed calculated using the formula (1) while a target diameterof the portion was set to the 150 mm glass rod target diameter.

A feed amount of the glass preform was set to 0 mm at the time pointthat the 200 mm length of preliminarily heated region passed through theheater middle position C, and the glass preform was fed by a feed amountof 1000 mm. During this time, a drawing speed v2 at a given position inthe usable region was calculated by the formula (1) using a diameter atan actual feed position of the preform, which is located at the heatermiddle position C. Another 200 mm length of region was further drawnafter drawing of the usable region. A diameter portion larger than 150mm of the tapered portion on the drawing start side was drawn at adrawing speed calculated using the formula (1) while a target diameterof the portion was set to the 150 mm glass rod target diameter. As aresult, as shown in FIG. 6, a difference between a maximum diameter anda minimum diameter in the diameter fluctuation of the drawn glass rodwas approximately 3.1 mm, and the diameter fluctuation of the drawnglass rod was larger than that of Example 1.

Comparative Example 2

A glass preform having tapered portions at both ends thereof, a 1000 mmlength usable region of the glass preform, a diameter of 161 mm at adrawing start position of the usable region, a diameter of 172 mm at adrawing ending position of the usable region, and a diameter in theusable region linearly varying in the longitudinal direction was drawnunder conditions where a heater length is 130 mm, a glass rod targetdiameter is a 150 mm, and a feed speed is 10 mm/min. No preliminarilyheated region on the drawing start side was set. Drawing referencedistances on the drawing start side and on the drawing ending side wereset to 35 mm and 53 mm determined from the data of FIG. 3A,respectively. The glass preform was set so that a drawing start positionof the usable region on the drawing start side is located below theheater middle position C by 35 mm and a drawing was started at a heatertemperature of 2050 degrees Celsius.

A feed amount of the glass preform at the heater middle position C whenstarting drawing was set to 0 mm From here, the glass preform was moved1018 mm. The feed amount of 1018 mm was determined from the usableregion length of 1000 mm+(the ending position drawing reference distanceof 53 mm−the start position drawing reference distance of 35 mm). Duringthis time, a diameter acquisition distance with respect to a givenposition on the preform was interpolated by multiplying an actual feedamount of the glass preform by 1000/1018. And, a drawing speed V2 at agiven position in the usable region was calculated by the formula (1)using a diameter acquired at the interpolated diameter acquisitiondistance from the measured diameter data of the glass preform. Inaddition, another 200 mm length of region was further drawn after adrawing of the usable region. A diameter portion larger than 150 mm ofthe tapered portion on the drawing start side was drawn at a drawingspeed calculated using the formula (1) while a target diameter of theportion was set to the 150 mm glass rod target diameter. As a result, asshown in FIG. 7, a large diameter fluctuation having a range of from +2mm to −5 mm was generated on the drawing start side, and the diameterfluctuation was larger than that of Example 1.

Example 2

A glass preform having tapered portions at both ends thereof, a 1000 mmlength usable region of the glass rod, a diameter of 168 mm at a drawingstart position, a diameter of 172 mm at a drawing ending position, and adiameter in the usable region varying in a wavy form in the longitudinaldirection was drawn under conditions where a heater length is 130 mm, aglass rod target diameter is a 150 mm, and a feed speed is 10 mm/min. Apreliminarily heated region on the drawing start side was set to 200 mmin length. Drawing reference distances with respect to a plurality ofpoints located on the glass preform in the longitudinal direction at aregular interval of 100 mm were set to 46, 44, 41, 38, 38, 39, 43, 46,49, 52 and 52 mm from the start end side of the glass preform, whichwere determined from diameters at the plurality of points and the dataof FIG. 3A. The glass preform was set so that a drawing start positionof the usable region on the drawing start side was located above theheater middle position C by 154 mm and a drawing was started at a heatertemperature of 2050 degrees Celsius. A diameter portion larger than 150mm of the tapered portion on the drawing start side was drawn at adrawing speed calculated using the formula (1) while a target diameterof the portion was set to the 150 mm glass rod target diameter.

A feed amount of the glass preform was set to 0 mm at the time pointthat the 200 mm length of preliminarily heated region passed through theheater middle position C, and the glass preform was moved 98 mm. Thefirst feed amount of 98 mm was determined from the interval of 100mm+(the drawing reference distance of 52 mm at the second point of theplurality of points−the drawing reference distance of 33 mm the firstposition of the plurality of points). During this time, a diameteracquisition distance with respect to a given position on the preformbetween the first and second position was interpolated by multiplying anactual feed amount of the glass preform by 100/98. And, a drawing speedwas calculated by the formula (1) using a diameter acquired at theinterpolated diameter acquisition distance from the measured diameterdata of the glass preform. The drawing was continued to the end of theusable region of the preform so that a drawing speed was sequentiallydetermined in the same way at each interpolation region between twopoints lying to next to each other among the plurality of points. Inaddition, another 200 mm length region was further drawn after thedrawing of the usable region. The diameter portion larger than 150 mm ofthe tapered portion on the drawing ending side was drawn at a drawingspeed calculated using the formula (1) while a target diameter of theportion was set to the 150 mm. As a result, as shown in FIG. 8, asignificantly reduced diameter fluctuation, in which a differencebetween a maximum diameter and a minimum diameter is approximately 0.9mm, could be obtained.

INDUSTRIAL APPLICABILITY

According to the present invention, a diameter fluctuation of a drawnglass rod in a longitudinal direction can be suppressed when drawing ata relatively large diameter reduction ratio.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

What is claimed is:
 1. A method of fabricating a glass rod by feeding arelatively large diameter glass preform into a furnace through a topportion thereof and drawing the glass preform from the furnace through abottom portion thereof so that the relatively large diameter glasspreform is drawn into a relatively small diameter glass rod, comprisingthe steps of: controlling a feed speed (V1) and a drawing speed (V2) ofthe glass preform so that a ratio (V2/V1) between the feed speed (V1)and the drawing speed (V2) becomes a value ((D/d)²) determined based ona diameter (D) of the glass preform and a target diameter (d) of theglass rod; setting the feed speed (V1) of the glass preform to aconstant value; and acquiring the diameter (D) of the glass preform fordetermining the drawing speed (V2) from a measured diameter data of theglass preform before being drawn at a diameter acquisition positiondefined with respect to a reference position of the furnace, wherein adistance from the reference position to the diameter acquisitionposition is defined so as to vary depending on a diameter fluctuation ofthe glass preform before being drawn in a longitudinal directionthereof.
 2. The method of claim 1, wherein in the step of acquiring thediameter (D), the distance from the reference position to the diameteracquisition position is defined so as to become equal to a drawingreference distance of the glass preform, and the drawing referencedistance is defined as a distance from the reference position of thefurnace to a specific position defined depending on a deformationcondition of a diameter decreasing region of the glass preform duringdrawing, and the drawing reference distance varies depending on thediameter before being drawn or the feed speed of the glass preform. 3.The method of claim 2, wherein drawing reference distances at first andsecond positions on the glass preform to be drawn in a longitudinaldirection thereof is determined based on a relation data between adiameter of the preform before being drawn and a measured drawingreference distance, and a drawing reference distance at a given positionbetween the first and second positions is determined by interpolatingthe determined drawing reference distances with respect to the first andsecond positions.
 4. The method of claim 2, wherein an amount of changeof the drawing reference distance with respect to a variation of thediameter before drawn or the feed speed of the glass preform differsdepending on a diameter reduction ratio, the diameter reduction ratiobeing defined as a ratio (d/D) between the diameter (D) before drawn andthe target diameter (d).
 5. The method of claim 4, wherein the diameterreduction ratio is within 60 to 95%.
 6. The method of claim 4, whereinthe glass rod target diameter (d) is set to equal to or more than 110mm.
 7. The method of claim 1, further comprising a step of starting toheat the glass preform before a drawing starting position of a usableregion in the glass preform reaches the reference position of thefurnace, wherein a distance between a position for starting to heat theglass preform and the drawing starting position of the usable region isset to equal to or more than a length of a heater in the furnace in alongitudinal direction and less than three times of the length.
 8. Anapparatus for fabricating a glass rod, comprising: a furnace; a feedingmechanism configured to feed a relatively large diameter glass preforminto a furnace through a top portion thereof; a drawing mechanismconfigured to draw the glass preform from the furnace through a bottomportion thereof so that the relatively large diameter glass preform isdrawn into a relatively small diameter glass rod; a controllerconfigured to control a feed speed (V1) of the glass preform by thefeeding mechanism and a drawing speed (V2) of the preform by the drawingmechanism so that a ratio (V2/V1) between the feed speed (V1) and thedrawing speed (V2) becomes a value ((D/d)²) determined based on adiameter (D) of the glass preform and a target diameter (d) of the glassrod, wherein the controller comprises: a setting unit configured to setthe feed speed (V1) of the glass preform to a constant value; anacquisition unit configured to acquire the diameter (D) of the glasspreform for determining the drawing speed (V2) from a measured diameterdata of the glass preform before being drawn at a diameter acquisitionposition defined with respect to a reference position of the furnace,wherein a distance from the reference position to the diameteracquisition position is defined so as to vary depending on a diameterfluctuation of the glass preform before being drawn in a longitudinaldirection thereof.